"Spooky" Action at a Distance

[Jamie123]

Here's what I don't get...

(I'm not an expert on quantum mechanics by the way. Maybe one day I'll find time to study it "properly" and put to bed these issues, but for right now please accept my observations as the witterings of a rank amateur.

OK here's the scenario: Alice and Bob, together at X, create a pair particles, each in a "coherent" state, with 50% chance of collapsing (if observed) into an up state or a down spin state. The two particles are entangled, so if Alice's collapses into an up state, then Bob's instantly collapses into the down state.

Alice and Bob then move to A and B, which might be several light-years apart. They still do not observe their particles, so they are still in 50/50 superposition and still entangled. Then at a point A' (in the future of A) Alice looks at her particle and collapses it into the up state. At that same instant, Bob's particle (at B') must collapse into the down state. However, that means that a "message" (of sorts) must have travelled instantly from A' to B', which defies the "rule" that information cannot travel faster than light.

Most YouTube videos at this point say "Weird, huh? Maybe faster-than-light communication is possible! Wouldn't that be cool?" before moving on to something else.

However if Bob does not also observe his particle, he will not know anything about its spin state. Also, even if he did look at his particle and find it in the down state, he would not know whether he caused it to collapse himself, or whether it had already collapsed due to what Alice had done. The effect would be the same either way.

Or to put it another way, Alice's action would have changed Bob's uncertainty from "aleatory" (uncertainty of chance) to "epistemic" (uncertainty of knowledge).

So the only way Alice could signal Bob faster-than light would be to have Bob sit looking at the box containing his particle, and when he notices his uncertainty of the box's content changing from aleatory to epistemic, he knows that Alice has signalled.

But how would he tell the difference? It is only when Alice and Bob come back together (at Y) and sees Alice's particle that he receives any information at all.

One might say that the uncertainty was always epistemic: that the spin-states of both particles were decided at X, and it is only Alice and Bob's knowledge which is uncertain. But this (i) makes the whole thing even more boring and mundane, and (ii) requires the "local hidden variables" theory, which was (as I understand it) disproved by John Stewart Bell.

So can "action at a distance" really have any impact on reality? If so, then how? And if not, is it really any more important than the number of angels that can dance on a pinhead?

Edited June 10, 2022 by Jamie123

[NeuroTypical]

Hm.  I'm dumber than most, but what if the experiment involved a billion particles and Alice making a billion measurements?  Bob places his double-slit plate thingy in the path of his billion particles, and does nothing else to observe his stuff, besides look at the pattern.  One would think at some point, the pattern would change, right when Alice starts observing, right?

[NeuroTypical]

And the impact on reality seems to be somewhere between "stress me the crap out" and "make it so anyone can guess my password". 

[Traveler]

7 hours ago, Jamie123 said:

Here's what I don't get...

.......

So can "action at a distance" really have any impact on reality? If so, then how? And if not, is it really any more important than the number of angels that can dance on a pinhead?

 

The difficult to understand answer is --- Yes!.  Now, I will try to make some sense with the example of the subatomic fermion particle we call a neutrino.  Initially the neutrino was theorized because observable nuclear reactions ended with the sum of it parts being less than the initial whole of all parts.  It was determined that the only way that such a thing could happen is if something was lost (that we never knew about in the first place).   Since the initial theory of the neutrino we have discovered a way to detect neutrinos - which is in essence to set up a reverse reaction minus the needed neutrino.  

In short a great deal has been added to our understanding since I studied particle physics some 55 years ago.  For example we have discovered that there are types or stages of neutrinos that change in time - but neutrinos are fermions that travel at the speed of light - but at the speed of light time is suppose to stop.  This means that there is a property of time that we have yet to discover.  Now, the theory of the day with neutrinos is that they are a "Dark Matter" element.  And yet no one has ever seen a neutrino (or even its trace in a bubble chamber). 

The point is this - if an angel or angels can dance on a pinhead the truth of it will eventually be discovered - thought you are correct the theory and argument over theory does little to reality.  What is truth can be discovered once we understand how to test for it.  It has always been my personal belief that there is no random event - just an event that has occurred outside our understanding --- Yet!  I am quite sure there are things about time and space (which we think we understand far more than sub atomic particles) that seem impossible at our understanding that are possible - when we understand reality beyond our current understanding.  As a retired scientist and engineer - the trick is knowing how to "test" what it is we think we know. 

 

The Traveler